Cirriform clouds (ie.clouds from the cirrus family) are found at high altitude, usually above 20,000 feet (6,000 metres). They are composed of ice crystals. Three types of cloud make up the group: cirrus, cirrostratus and cirrocumulus.

Cirrus itself is very common in the British Isles and throughout most of the world. It is thin, wispy and white in appearance, and its name, coming from the Latin work for 'tuft of hair' gives a good description of the cloud. Another name for the cloud, 'mares tails', also conjures up an accurate image. Cirrus may be hooked or straight depending on the airflow aloft. Sometimes it comes as a very dense patch which is left over from the 'anvil' cloud of a cumulonimbus which has disappeared. On other occasions, cirrus may be quite extensive when associated with a jet stream - the cloud can then be seen moving across the sky despite its great altitude. Aircraft condensation trails are a form of man-made cirrus. They can sometimes be seen in 'historical' films, to the delight of film buffs who enjoy spotting technical inaccuracies.

Cirrostratus is a fairly uniform sheet f thin cloud through which the sun or moon can be seen. Sometimes, if the cloud is thin, a bright ring of light (called a halo) surrounds the sun or moon. A layer or cirrostratus is often an indication of a deterioration in the weather.

Cirrocumulus is often present in small amounts along with cirrus, but rarely does it dominate the sky. On those occasions when it is widespread a beautiful spectacle is created, especially at sunset. The individual clouds appear very small - often tiny rows of roughly spherical pear-like cloud elements. Sometimes they occur in undulating patterns like tiny ripples.

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle